The invention relates to an apparatus and method for processing ceramic materials. In particular, this invention relates to an apparatus and method for processing ceramic materials involving a combination of microwave and conventional heating, and more particularly to an apparatus and method for controlling the field density of the microwave power through slotted waveguides to uniformly distribute the power throughout the ceramic material, maximizing power absorption by the ware and minimizing power absorption by its surroundings.
Conventional heating used in the manufacturing of ceramic materials typically utilizes radiative gas flame, electric resistance heating, and the like. Utilization of conventional heating typically results in a thermal differential within the ceramic material. This differential is due, in part, to the fact that radiant heating is applied only to the surface of the material and it relies on the thermal conductivity of the material, typically poor, to transmit the thermal energy beneath the surface and into the interior of the piece. In other words, conventional heating involves heat transfer that is predominantly achieved by radiation or convection to the surface followed by conduction from the surface into the interior of the ceramic body. If a core-surface thermal differential develops that is too great, internal cracking and distortion of the ceramic material can occur. Fast firing further exacerbates this problem of poor heat transfer, and ultimately cracking.
Additionally, the presence of a core-surface thermal gradient can also result in uneven sintering, specifically surface sintering prior to, and at a faster rate than, interior sintering. As a result, the ceramic material may exhibit non-uniform properties.
Solutions to these problems have been proposed which involve reducing the rate of heating or allowing lengthy holds at certain temperatures. Each of these solutions allows heat energy to be conducted into the core of the ceramic material, which in turn, allows the temperature of the core of the ceramic material to xe2x80x9ccatch upxe2x80x9d with that of the surface, thereby minimizing the surface/core temperature differential. Unfortunately however, the theoretical limits of conventional heating typically result in slow heating rates for most ceramic materials, the exception being ceramic pieces exhibiting small dimensions.
Microwave heating of ceramics has alternatively been successfully used to fire ceramic materials. In comparison with conventional heating, microwave heating involves depositing energy directly within the ceramic material in accordance with a volumetric heating mechanism. More specifically, the utilization of microwave energy involves delivering energy to the entire cross section of the ceramic article, rather than to the article surface. Although microwave heating of ceramic materials is much faster than conventional heating because of this volumetric heating, when used alone, it, like conventional heating, results in the ceramic article exhibiting a thermal differential, albeit an opposite thermal differential with the core of the ceramic article exhibiting a higher temperature than that of the surface. Specifically, as the ceramic materials are heated with microwaves to high temperatures, the interior of the ceramic article very rapidly begins to absorb substantial amounts of microwave energy; this effect is known as thermal runaway.
Although the surface is heated along with the core of the ceramic material, the surface rapidly loses much of its heat energy to the surroundings, which is typically cooler than the ceramic material average temperature. As the core starts to preferentially absorb the microwave energy this thermal runaway phenomenon becomes self-propagating. Simply stated, as the temperature of the ceramic material increases, the heat losses become greater, and the magnitude of the core-surface thermal differential increases, again leading to thermal stress within, and ultimately cracking of, the ceramic article.
In addition to heat losses from the surface of the ceramic article, non-uniformity of the microwave distribution within the kiln and non-uniform material properties of the ceramic article lead to differential absorption of the microwave energy by the ceramic article, and contribute to the microwave heating thermal differential.
Hybrid microwave/conventional heating or microwave assisted heating has been proposed as an alternative to overcome the problems of conventional-only and microwave-only heating. In microwave assisted heating involving both microwave and conventional heating, the volumetric heating provided by the microwaves heats the components, while the conventional heating provided by gas flame or electric resistance heating elements minimizes heat loss from the surface of the components by providing heat to the surface and its surroundings. This combination of hybrid heating can result in heating that avoids thermal profiles associated with conventional-only and microwave-only firing. As a result, thermal stresses can be reduced and or minimized and thus the ceramic articles can be heated more rapidly.
Although various methods of implementation have been proposed, it can be difficult to coordinate the respective microwave and conventional power inputs to achieve optimal uniform heating of the ceramic article. Variations on microwave-assisted ceramic firing standard control methods are disclosed in PCT Applications WO 95/05058 and WO 93/12629 and U.S. Pat. No. 5,191,183. These documents generally disclose methods of independently controlling the quantities of heat generated in the ceramic article by the microwave energy and radiant heat by measuring the ambient temperature within an enclosure containing the ceramic article. Based on, and in response to, this ambient temperature measurement, the heat generated in the ceramic article is controlled by one or both of the microwave energy or radiant heat. Although this type of control method is an improvement over prior conventional control methods, the non-uniform mixing of kiln gases and the effects of chemical reactions that occur within the ceramic material make it difficult to accurately predict the ceramic article surface and internal temperatures.
Heating uniformity is of paramount importance in most industrial heat treating applications. Typically, multiple ceramic articles are stacked in layers within the kiln to increase productivity. This increases the importance of uniform distribution of suitable amounts of thermal energy within the kiln to assure that each ceramic piece is fired properly, thus avoiding burning, cracking or other undesirable results. Uniform distribution is particularly critical in the case of relatively fragile green thin-walled ceramic honeycomb structures where minimal heating stresses must be maintained in order to produce crack-free ware in commercial quantities and acceptable process yields.
Methods to improve the dispersion of microwave power include the use of stirrers at the end of waveguides or in other parts of the microwave cavity. Although, stirrers placed in front of microwave ports help to randomize the power distribution, they are associated with other problems such as high reflectance and the need to add tuners to the waveguide to minimize the potential damage of this reflected power, and the need for maintenance with moving parts in a hot environment.
Alternatives to the use of stirrers include the use of multiple waveguides and slotted waveguides in an effort to distribute the power more evenly. However, to date, the art lacks a kiln configuration which provides for the desired uniform distribution of the microwave heating power which enables increased power absorption by each article or part of the ware.
U.S. Pat. No. 4,164,742 relates to a slotted-waveguide type beam-forming antenna array as commonly employed in radar systems. The purpose of this waveguide array is to produce multiple beams from a slot array aperture. Two waveguides having coupling slots are joined at a common boundary to form discrete beams for radar applications. Beam splitting would not be useful to provide a uniform heating pattern in a kiln for processing ceramics.
Accordingly, one aspect of the present invention is to provide an apparatus and method for uniformly dispersing the microwave power in a combined microwave and conventional heating system utilized in the heating of ceramics that overcomes the shortcomings of the aforementioned systems.
In accordance with another aspect of the present invention, there is provided a method for processing a ceramic material, including a plurality of ceramic articles, which comprises placing the ceramic material in a microwave heating apparatus having a microwave cavity. The ceramic material is subjected to a combination of microwave radiation and conventional heat. The uniformity of the processing of the ceramic material is controlled so as to avoid deformations and cracking by applying the microwave radiation to the ceramic material through at least one and preferably an array of slotted waveguides, most desirably branched slotted waveguides. The waveguide or waveguides are configured to substantially uniformly distribute the microwave radiation throughout the microwave cavity.
The uniformity of distribution is such that the maximum deviation between the microwave power delivered to a ceramic article in a first position within the cavity and that delivered to a ceramic article in a second position within the cavity is no greater than about 25%, as measured in a static system. Further, this uniformity of power distribution is maintained under conditions wherein the power input to the ceramic ware within the microwave cavity is maintained at a level equal to or exceeding 15xc3x9710xe2x88x923 mW/m2.
In accordance with another aspect of the present invention, there is provided an apparatus including a conventional heat source in communication with a microwave cavity. A microwave power source is in communication with the microwave cavity through at least one slotted waveguide. The at least one slotted waveguide substantially uniformly distributes the microwave radiation throughout the microwave cavity such that the power into a ceramic material in the microwave cavity is above about 15xc3x9710xe2x88x923 mW/m2 and the maximum deviation between the microwave power supplied to the ceramic material in a first defined area and that supplied to the ceramic material in a second defined area within the cavity is no greater than about 25%, as measured in a static system.